Mary Rich, the mother of the Democratic operative Seth Rich, at a news conference in Washington in 2016. His family has spent years fighting misinformation about his death.CreditCreditMichael Robinson Chavez/The Washington Post, via Getty Images

The special counsel’s report confirmed this week that Seth Rich, a young Democratic National Committee employee whose unsolved killing became grist for a right-wing conspiracy theory, was not the source of thousands of internal D.N.C. emails that WikiLeaks released during the 2016 presidential race, officially debunking a notion that had persisted without support for years.

Tucked amid hundreds of pages of the report’s main findings, the special counsel, Robert S. Mueller III, took aim at WikiLeaks and its founder, Julian Assange, for falsely implying that Mr. Rich was somehow involved in the dissemination of the emails, an act that aided President Trump’s campaign.

“WikiLeaks and Assange made several public statements apparently designed to obscure the source of the materials that WikiLeaks was releasing,” according to the report, which showed that WikiLeaks corresponded with the true source of the leaked emails — Russian hackers — after Mr. Rich’s death.

The confirmation comes after years of anguish for Mr. Rich’s family, who fought attempts to politicize and spread misinformation about his killing, which is believed to have happened during a bungled robbery attempt.

The theory linking Mr. Rich to the email leak took root in conservative circles and was cited by prominent conservatives like Newt Gingrich and right-wing commentators like Rush Limbaugh, Sean Hannity and Alex Jones of Infowars. WikiLeaks offered a $20,000 reward for information about Mr. Rich’s killing, fueling speculation that he was the source. Fox News also published an article, which the network later retracted, suggesting that Mr. Rich was killed in retaliation for having leaked the emails.

In a statement, Mr. Rich’s brother, Aaron Rich, responded to the special counsel’s report, saying it provided “hard facts that demonstrate this conspiracy is false.”

“I hope that the people who pushed, fueled, spread, ran headlines, articles, interviews, talk and opinion shows, or in any way used my family’s tragedy to advance their political agendas — despite our pleas that what they were saying was not based on any facts — will take responsibility for the unimaginable pain they have caused us,” he said.

On July 14, WikiLeaks received an encrypted file from Russian hackers, according to the report. The organization published thousands of internal D.N.C. emails later that month, just days ahead of the Democratic convention.

“That chronology is damning,” Mike Gottlieb, a lawyer for Aaron Rich, said, pointing out that Mr. Rich had already been killed when the file was sent.

In statements beginning that summer, Mr. Assange and WikiLeaks “implied falsely” that Mr. Rich had been the source of the emails, the special counsel’s report said.

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Julian Assange arriving at court in London. He was arrested this month on charges unrelated to to WikiLeaks’ role in Russia’s operations to sabotage the 2016 presidential election.CreditVictoria Jones/Press Association, via Associated Press

In addition to the $20,000 reward, Mr. Assange said in an interview that he was interested in Mr. Rich’s death because “we’re very interested in anything that might be a threat to alleged WikiLeaks sources,” according to the report.

Even after intelligence officials announced that Russia was behind the email hacking, Mr. Assange continued to deny Russian involvement and told a congressman that the D.N.C. hack was an “inside job,” according to the report.

“Assange did untold damage to a grieving family in order to try and hide his work with Russian intelligence,” Brad Bauman, a former spokesman for Mr. Rich’s family, said in a statement after the release of the report.

“He is a monster, not a journalist,” he added.

Mr. Assange, whose release of secret government documents has spurred debate about press freedom issues, spent seven years holed up in the Ecuadorean Embassy in Britain before he was arrested this month on a charge of conspiring to hack into a Pentagon computer network in 2010. The charge is not related to WikiLeaks’ role in Russia’s operations to sabotage the election.

Though Mr. Rich’s killing remains unsolved, the special counsel’s findings could begin to ease the pain for his family, who have described the anguish of having to watch his life and death be treated like a “political football.”

His parents, Joel and Mary Rich, sued Fox News last year, claiming that the network’s coverage helped fuel damaging rumors about their son. A judge, while expressing sympathy for the family, dismissed the case because the parents had not been personally defamed by the story, despite the fact that it included “false statements or misrepresentations.” The judge noted that Mr. Rich could not be defamed by the story under New York law because he was dead.

Aaron Rich is also suing for defamation over theories that falsely implicate him, purporting that he helped steal the data and cover up his brother’s killing. He accepted an apology and retraction from The Washington Times last year as part of a settlement.

Mr. Gottlieb, his lawyer, said that while it was gratifying for his client to see the Mueller report lay to rest any lingering conspiracy theories, Mr. Rich was still grappling with the loss of his brother, who was the best man at his wedding.

“The picture of Seth and Aaron at Aaron’s wedding is one a bunch of conspiracy theorists have used as memes,” he said.

“Aaron has been called a traitor,” Mr. Gottlieb added. “He is being called every imaginable awful name you can think of and he’s never had the opportunity to grieve.”

In his statement, Aaron Rich vowed to continue fighting.

“We will continue to pursue justice for Seth’s murderers,” he said, “as well as those who used his murder to advance their personal or political agendas.”